Progesterone receptor modulators can be prepared by thionation of carbonyl compounds. The thionation of benzoxazin-2-ones using either 2,4-bis(4-methoxyphenyl)-1,3-dithia-2,4-diphosphetane-2,4-disulfide (Lawesson's reagent) or phosphorous pentasulfide is known (U.S. Pat. No. 6,436,929). See, Scheme 1.

Such compounds are useful for contraception, hormone replacement therapy, synchronization of estrus, and in the treatment of conditions including hormone neoplastic diseases, adenocarcinomas, and carcinomas.
However, certain impurities formed during thionation are difficult to remove. What is needed in the art are methods for reducing or eliminating the formation of impurities.